The present invention pertains to the field of flat panel display screens. More specifically, the present invention relates to the field of flat panel field emission display screens.
Flat panel field emission displays (FEDs), like standard cathode ray tube (CRT) displays, generate light by impinging high energy electrons on a picture element (pixel) of a phosphor screen. The excited phosphor then converts the electron energy into visible light. However, unlike conventional CRT displays which use a single or in some cases three electron beams to scan across the phosphor screen in a raster pattern, FEDs use stationary electron beams for each color element of each pixel. This requires the distance from the electron source to the screen to be very small compared to the distance required for the scanning electron beams of the conventional CRTs. In addition, FEDs consume far less power than CRTs. These factors make FEDs ideal for portable electronic products such as laptop computers, pocket-TVs, personal digital assistants, and portable electronic games.
One problem associated with the FEDs is that the FED vacuum tubes may contain a minute amount of contaminants which can become attached to the surfaces of the electron-emissive elements, faceplates, gate electrodes (including dielectric layer and metal layer) and spacer walls. These contaminants may be knocked off when bombarded by electrons of sufficient energy. Thus, when an FED is switched on or switched off, there is a high probability that these contaminants may form small zones of high pressure within the FED vacuum tube. In addition to the fact that the gate is positive with respect to the emitter, the presence of the high pressure facilitates electron emission from emitters to gate electrodes. The result is that some electrons may strike the gate electrodes rather than the display screen. This situation can lead to overheating of the gate electrodes. The emission to the gate electrodes can also affect the voltage differential between the emitters and the gate electrodes. In addition, as the electrons jump the gap between the electron-emissive elements and the gate electrode, a luminous discharge of current may also be observed. Severe damage to the delicate electron-emitters may also result. Naturally, this phenomenon, generally known as xe2x80x9carcing,xe2x80x9d is highly undesirable.
Conventionally, one method of avoiding the arcing problem is by manually scrubbing the FED vacuum tubes to remove contaminant material. However, it is difficult to remove all contaminants with that method. Further, the process of manual scrubbing is time-consuming and labor intensive, unnecessarily increasing the fabrication cost of FED screens.
In addition to the problem of arcing produced by pressure increases associated with the electron induced desorption of contaminant species from the faceplate and other surfaces, there is also a problem involved with the distribution of the particles after desorption. Ideally, the desorbed contaminants are trapped by a getter in the tube; however, in practice, the desorbed species may be adsorbed and desorbed many times from various surfaces before being gettered, and when the desorbed contaminant species are deposited non-uniformly on the emitter surfaces, the display uniformity is affected.
The intensity of the emission current from an emitter element is a function of the work function at the surface of the emitter. Adsorbed chemical species may either increase or decrease the work function. For example, methane molecules adsorbed on the tip of a molybdenum emitter will enhance emission by reducing the work function, whereas adsorbed oxygen will reduce emission by increasing the work function.
Since FEDs are vacuum devices, the faceplate must be supported by spacer walls if it is of a significant size. The presence of the spacer walls produces local variations in the distribution of redeposited desorbed species from.the faceplate, and this non-uniformity may appear as banding in the display. Accordingly, the present invention provides an improved method of removing contaminant particles from the FED screen. The present invention also provides for an improved method of operating field emission displays to prevent gate-to-emitter currents during turn-on and turn-off. These and other advantages of the present invention not specifically described above will become clear within discussions of the present invention herein.
The present invention provides for a method of removing contaminant material in newly fabricated field emission displays. According to one embodiment of the present invention, contaminant particles are removed by a conditioning process, which includes the steps of: a) driving an anode of a field emission display (FED) to a predetermined voltage; b) slowly increasing an emission current of the FED after the anode has reached the predetermined voltage; and c) providing an ion-trapping device for catching the ions and contaminants knocked off by emitted electrons. In this embodiment, by driving the anode to the predetermined voltage and by slowly increasing the emission current of the FED, contaminant particles are effectively removed without damaging the FED.
The present invention also provides for a method of operating FEDs to prevent gate-to-emitter current during turn-on and turn-off. In this embodiment, the method includes the steps of: a) enabling the anode display screen; and, b) enabling the electron-emitters a predetermined time after the anode display screen is enabled. In this embodiment, by allowing sufficient time for the anode display screen to reach a predetermined voltage before the emitter is enabled, the emitted electrons will be attracted to the anode. In this way, gate-to-emitter current is effectively eliminated when an FED is turned on. In the present embodiment, the anode display screen is enabled by applying a predetermined high voltage to the display screen, and the electron-emitters are enabled by driving appropriate voltages to the gate electrodes and emitter electrodes of the FED.
In yet another embodiment of the present invention, the method of operating field emission displays to prevent gate-to-emitter current includes the steps of: a) disabling the emitters for a predetermined time; and, b) disabling the anode display screen after the electron-emitters are disabled. In this embodiment, by allowing sufficient time for the electron-emitters to be disabled before disabling the anode display screen, all remaining electrons will be attracted to the anode. In this way, gate-to-emitter current is eliminated during a turn-off sequence of the FED. In the present embodiment, the anode display screen and electron emitters are disabled by switching off the votage source and allowing the potential to decay to ground.
A further embodiment of the present invention includes a method of operating a field emission display so that the flux of contaminant species produced by electron induced desorption during a conditioning period results in a uniform distribution of contaminant species on the emitters.
Embodiments of the present invention include the above and further include a method of operating a field emission display, the method comprising the steps of: providing the field emission display with electron-emissive elements for emitting electrons, a gate electrode for controlling electron emission from the electron-emissive elements, and a display screen for collecting the electrons; enabling the display screen to establish a voltage differential between the display screen and the electron-emissive elements; and following enabling of the display screen, enabling the gate electrode by delaying substantial electron emission from the electron-emissive elements until the voltage differential has been established to direct the electrons towards the display screen and to substantially prevent the electrons from striking the gate electrode.
Embodiments of the present invention further include a field emission display device comprising: a baseplate; a plurality of electron-emissive elements on the baseplate; a gate electrode on the baseplate for controlling electron emission from the electron-emissive elements; a display screen spaced from the baseplate and configured for collecting electrons emitted from the electron-emissive elements to generate an image thereon; and a control circuit configured to control a flow of electrons to the electron-emissive elements, the control circuit allowing a voltage differential to be established between the display screen and the electron-emissive elements prior to substantial electron emission from the electron-emissive elements to prevent substantial gate-to-emitter current during turn on of the field emission display device.
Another method embodiment used in conjunction with the field emission display device described above is used for equalizing readsorption of contaminant species, the method comprising the steps of: determining the angular distribution of the desorbed species; determining the anticipated accumulation of the desorbed species at the emitter.-sites; determining a time average current emission for each of the emitter sites wherein the time integrated flux of contaminant species is substantially the same at each of the emitter sites; and, driving each emitter site with the determined emission current.